"A Cup of Coffee, A Sandwich and You" by Joseph Meyer - only she adds her own lyrics: "A cup of arsenic, a spider, some glue/A lizard's gizzard, an eel head or two."
He's trick-or-treating as an ugly witch - thus inflaming her with jealousy. She thinks he's a real witch, and - as her own magic mirror tells her - Bugs is far uglier than she is.